The present invention relates to a method for producing high strength steels by so-called TMCP (Thermomechanical Control Process), which exhibit excellent properties after warm working.
Steels to be used to offshore structural materials or the like are required to have high strength and high toughness, and this kind of steel has been conventionally produced by normalizing or quenching-tempering treatment.
Recently, techniques such as controlled rolling or accelerated cooling for producing steel plates of heavy thickness, have been advanced and distributed as TMCP, and applied to the offshore structural steels.
The steels of TMCP type are imparted with the high strength and high toughness by rolling at ranges of low temperatures of austenite or at (.alpha.+.gamma.) intercritical range, otherwise by controlling transformation from austenite to ferrite by the accelerated cooling after rolling.
The steels for the offshore structures are subjected to bending when setting up, and generally steels of small thickness or low strength are performed with cold working, and steels of thickness are done with warm working.
If the steel of TMCP type were re-heated up to the austenite range for the warm working, it would be more deteriorated in properties than conventional materials. Although, in the cold working, no problem arises about the properties, but as it has been possible to produce steels of high strength and heavy thickness, a problem occurs that the cold working could not be performed because of pressing ability.
In view of such a problem, a warm working process which performs processing after having heated at .alpha. high temperature range or (.alpha.+.gamma.) intercritical range, has been applied to TMCP steels of high strength or heavy thickness, and there have been many proposals for this technique. However, there has never yet been a proposal which has been studied, including mechanical properties after the warm working process.